For renewable electric power generated by wind or solar farms to make inroads into the US electric utility market, new technologies for electrical energy storage (EES) are needed that are efficient, cost effective, fast, reliable, scalable, and broadly deployable. The EES storage technologies can generally be classified into: 1) direct storage of electrons, e.g., in supercapacitors, 2) conversion of electric energy into mechanical energy, e.g., kinetic energy in flywheels or into potential energy in pumped hydropower, which currently accounts for 99% of the world-wide installed EES capacity, or compressed air energy storage (CAES), and 3) conversion of electric energy into chemical energy, e.g., in batteries, flow batteries, or unitized regenerative fuel cells (URFCs). The direct storage of electrons in supercapacitors is suitable only for short bursts for power management, while pumped hydropower and CAES are highly site specific.
Regenerative fuel cells, such as URFCs, circulate material in the form of a fluid for reacting the charge material in a cell to generate electricity from an electrochemical reaction between anode and cathode charge materials. In contrast to conventional fuel cells such as commonly employed in automobiles, portable electronics, and flashlights, URFCs do not need continuous feed of the anode and cathode materials to perform. Rather, they produce these materials in the same device when excess electricity is available and store them to use when electric production is needed. Thus, unitized regenerative fuel cells have the added property that the electrochemical reaction is reversible to return the products of power generation to an original state. In other words, like batteries, URFCs are electric power storage devices, with the key difference that the reactant storage units are distinct from the electrochemical cell.
The variable and intermittent nature of renewable electric power from solar or wind sources calls for the development of cost-effective, modular, grid-scale, electric power storage systems that are broadly deployable and geographically independent. Electrochemical energy storage, including batteries and regenerative fuel cells, is of the foremost interest in this regard, including: 1) secondary batteries, i.e., the sodium-sulfur battery, the lead-acid battery, the Ni—Cd battery; 2) flow batteries such as the vanadium redox battery and the zinc bromine battery; and 3) regenerative H2—O2 proton-exchange membrane (PEM) fuel cell. However, none of these yet approach the cost and cycle life of, for example, pumped hydropower at <$100/kWh and >5000 cycles, which is a key industry target.